X-ray, spectroscopic and computational studies of the tautomeric structure of a new hydrazone of 5-nitrosalicylaldehyde with indole-3-acetic hydrazide

A new hydrazone of 5-nitrosalicylaldehyde with indole-3-acetic hydrazide (NSIAH) has been obtained and its structure has been studied by X-ray, FT-IR and CPMAS in the solid as well as 1H, 13C and 15N NMR in the CH3CN-d3 and DMSO-d6 solutions. The hydrazone crystallises in the Pbca space group from the orthorhombic system with a = 9.5435(7) Å, b = 11.0932(8) Å and c = 29.672(2) Å. According to Hirsfeld surface analysis, the whole structure is dominated by H···O interactions (ca. 32.2%) and by H···H interactions (ca. 30%). The other important contributions come from: H···C interactions 19.3%, C···C interactions (π...π electrons; ca. 6.6%) and H···N interactions (also ca. 5.9%). The crystal structure has provided clear evidence that the hydrazone exists in the imine (hydroxy) form in which one intramolecular O–H···N hydrogen bond exists. The existence of this tautomeric form is well reflected by the presence of characteristic bands in the FT-IR spectrum of NSIAH. In CH3CN-d3 solution the imine (hydroxy) form with the intramolecular O–H···N hydrogen bond is preserved, whereas in DMSO-d6 this bond is partially broken and stabilized by the intermolecular O–H···O ones with the solvent molecule. As follows from the PM5 calculations of the NSIAH structures solvated by three DMSO-d6 molecules, structure A including the intramolecular O-H···N hydrogen bond and structure B including only intermolecular hydrogen bonds, are energetically equivalent.